compiler: initialize local variable in lower_method_expression

Fixes https://gcc.gnu.org/PR114463

Change-Id: I4d0a020dc5c3ae449cdc0007a288b1919b398f8f
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/574476
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

go/expressions.cc

@@ -9059,7 +9059,7 @@ Selector_expression::lower_method_expression(Gogo* gogo)
 
   Named_type* nt = type->named_type();
   Struct_type* st = type->struct_type();
-  bool is_ambiguous;
+  bool is_ambiguous = false;
   Method* method = NULL;
   if (nt != NULL)
     method = nt->method_function(name, &is_ambiguous);